Hydraulic units of the above-mentioned type are supplied to the vehicle maker either prefilled, or in a condition not filled with brake fluid. Because the wheel outlet valves in automotive vehicle brake systems operating according to the recirculation principle lead into so-called secondary circuits which correspond to the pump suction conduit with a low-pressure accumulator connected thereto, special bleeding and filling provisions are necessary in order to protect also the electrically and hydraulically operable valves which are isolated in the area of the secondary circuit against air inclusions, and to fill them with brake fluid in an operationally reliable manner.
In this respect, it is an advantage for the vehicle maker that the filling operation of the entire brake circuit is exclusively restricted to the primary circuit, i.e., the hydraulic area between the braking pressure generator and the pressure fluid connection which is open in the direction of the wheel brake. Therefore, the manufacturers of brake systems frequently supply the basic function components integrated in a hydraulic unit to vehicle makers in a prefilled condition.
Generic European patent application No. 0 482 367 discloses a hydraulic unit for an automotive vehicle brake system which operates according to the recirculation principle. The hydraulic unit is connected to a pump assembly by way of a connecting line. To fill the hydraulic unit with brake fluid, a feed bore is arranged on the hydraulic unit which opens into the connecting line upstream of a pair of non-return valves that close in the direction of the pump assembly. From the feed bore, the brake fluid propagates by way of one of the two non-return valves up to a portion of a suction conduit of the return pump which is isolated from the outlet valves by a non-return valve. Ports of the hydraulic unit lead from the outlet valves to several wheel brakes. Consequently, the portion of the suction conduit which is positioned between the outlet valves and the non-return valve that is downstream of the return pump cannot be prefilled through the feed hole.
An object of the present invention is to prepare a hydraulic unit of the above indicated type already to such effect that a most simple prefilling of the hydraulic unit is achieved by least possible structural and functional efforts.
According to the present invention, this object is achieved for a hydraulic unit of the above-mentioned type by an appropriate process of prefilling the hydraulic unit.